Kondo Cloud Research Articles

Quantum dot with ferromagnetic leads: A density-matrix renormalization group study

A quantum dot coupled to ferromagnetically polarized one-dimensional leads is studied numerically using the density-matrix renormalization group method. Several real space properties and the local density of states at the dot are computed. It is shown that this local density of states is suppressed by the parallel polarization of the leads. In this case we are able to estimate the length of the Kondo cloud, and to relate its behavior to that suppression. Another important result of our study is that the tunnel magnetoresistance as a function of the quantum dot on-site energy is minimum and negative at the symmetric point.

Two stages of Kondo effect and competition between RKKY and Kondo in Gd-based intermetallic compound

The magnetic behavior of Gd-based intermetallic compound ( Gd 2 Al ( 1 - x ) Au x ) in the form of the powder and needle, is investigated. All the samples are an orthorhombic crystal structure. Only the compound with x = 0.4 shows the Kondo effect (other compounds have a normal behavior). Although, for the compound in the form of powder, with x = 0.4 , the susceptibility measurement χ ( T ) shows two different stages. Moreover for ( T > T K 2 ) a fall of the value of χ ( T ) is observable, which indicates a weak presence of ferromagnetic phase. About the two stages of Kondo effect, we observe at the first ( T K 1 ) an increase of χ ( T ) and in the second stage ( T K 2 ) a new remarkable decrease of χ ( T ) , ( T K 1 > T K 2 ) . For the sample in the form of needles, the first stage is observable only under high magnetic field. This first stage could be corresponds to a narrow resonance between Kondo cloud and itinerant electron. The second stage, which is remarkably visible for the sample in the form of the powder, can be attribute to a complete polarization of Kondo cloud. Observation of these two Kondo stages could be due to the weak presence of RKKY contribution.

Anisotropic Kondo lattice without Nozieres exhaustion effect

The properties of layered Anderson/Kondo lattices with metallic electrons confined in 2D xy planes and local spins in insulating layers forming chains in z direction are studied. Each spin possesses its own 2D Kondo cloud, so that the Nozieres’ exhaustion problem does not arise. The excitation spectrum is gapless both in charge and spin sectors. Possible experimental realizations of the model are briefly discussed.

Controlled dephasing of a quantum dot in the Kondo regime.

Kondo correlation in a spin polarized quantum dot (QD) results from the dynamical formation of a spin singlet between the dot's net spin and a Kondo cloud of electrons in the leads, leading to enhanced coherent transport through the QD. We demonstrate here significant dephasing of such transport by coupling the QD and its leads to potential fluctuations in a nearby "potential detector." The qualitative dephasing is similar to that of a QD in the Coulomb blockade regime in spite of the fact that the mechanism of transport is quite different. A much stronger than expected suppression of coherent transport is measured, suggesting that dephasing is induced mostly in the "Kondo cloud" of electrons within the leads and not in the QD.

Correlations between Kondo clouds in nearly antiferromagnetic Kondo lattices

Abstract We discuss a novel fluctuational mechanism explaining the physics of nearly antiferromagnetic Kondo lattices (KL). The effective action for KL model is expressed in terms of Bose operators responsible for paramagnetic excitations and semi-bosonic fields describing the dynamic Kondo clouds created by conduction electrons around local spin. The gauge invariant resonance valence bond theory of interacting Kondo clouds describes the spin liquid with strong critical fluctuations imitating itinerant fluctuation magnetism of Moriya type.

Kondo screening cloud effects in mesoscopic devices

We study how finite size effects may appear when a quantum dot in the Kondo Coulomb blockade regime is embedded into a mesoscopic device with finite wires. These finite size effects appear when the size of the mesoscopic device containing the quantum dot is of the order of the size of Kondo cloud and affect all thermodynamic and transport properties of the Kondo quantum dot. We also generalize our results to the experimentally relevant case where the wires contain several transverse modes/channels. Our results are based on perturbation theory, Fermi liquid theory and slave boson mean field theory.

Transport through quantum dots in mesoscopic circuits.

We study the transport through a quantum dot, in the Kondo Coulomb blockade valley, embedded in a mesoscopic device with finite wires. The quantization of states in the circuit that hosts the quantum dot gives rise to finite size effects. These effects make the conductance sensitive to the ratio of the Kondo screening length to the wires length and provide a way of measuring the Kondo cloud. We present results obtained with the numerical renormalization group for a wide range of physically accessible parameters.

Spin fluctuation and persistent current in a mesoscopic ring coupled to a quantum dot

We investigate the persistent current influenced by the spin fluctuations in a mesoscopic ring weakly coupled to a quantum dot. It is shown that the Kondo effect gives rise to some unusual features of the persistent current in the limit where the charge transfer between two subsystems is suppressed. Various aspects of the crossover from a delocalized to a localized dot limit are discussed in relation with the effect of the coherent response of the Kondo cloud to the Aharonov-Bohm flux.

Formation of Kondo clouds in U(Sn1−xMx)3 compounds (M = Al, Ga, Ge)

USn3 has an unusually high γ value of 169 mJ mole−1 K−2 in comparison with its neighboring compounds UAl3, UGa3, USi3, and UGe3. To study the formation of this high γ value we prepared pseudobinary U(Sn1−xMx)3 compounds (M = Al, Ga, Si, Ge, and x = 0, 0.1, 0.25, 0.5). From X-ray diffraction, specific-heat and susceptibility measurements we conclude that the high γ value is not only due to f-ligand hybridization, but also to the formation of Kondo clouds (40%).

Superconducting and magnetic properties of Y 1− xU xRh 4B 4

The heat capacity, magnetic susceptibility and magnetization of Y 1− x U x Rh 4B 4 were determined within the range from x = 0 to x = 0.65. The influence of the local magnetic moments of the U atoms (d U-U = 5.27 A ̊ ) in the CeCo 4B 4 structure of the tetragonal RERh 4B 4 was studied. A.c. susceptibility measurements showed a rather large value for the T c depression of d T c/d x = −1.88 K per at. % U in the lower concentration range x = 0.0 to x = 0.06. In comparison with the (Bardeen-Cooper-Schrieffer) BCS theory and the Abrikosov-Gorkov model for dilute magnetic impurities, the decrease in the jump in specific heat at T c vs. the depression of T c is rather strong in these samples, so the existence of a demagnetizing Kondo cloud around the U moments seems likely. In the samples containing more U, the magnetization and d.c. susceptibility showed a tendency towards magnetic behaviour, but no evidence for long-range magnetic order was found.

Correlated Fermi-Liquid State Formed with Overlapping Kondo Clouds

A simple variational description is given on the nature of overlapping Kondo clouds, which compensate the localized f-spins in the Kondo lattice. It is shown explicitly that the f-electrons participate in forming a “large Fermi surface”, although they are strictly localized. The resulting correlated Fermi liquid, i.e., heavy fermions, is examined with the variational Monte Carlo method and is compared with the Gutzwiller approximation. The relation to the periodic Anderson model is also discussed in this connection.